Q1 - In what order are files played in a folder? From googling around it seems like the order depends on the ID3 tags, but which tags exactly? Is it track number, or song title (in alphabetical order?).

Q2 - My audiobook plays out of order if I start it from the folder list, so I used a playlist instead. Works fine, until I try to set a bookmark. If I resume from a bookmark, the playlist is ignored and it goes back to the folder order again. Is that normal? I'm using firmware 2.59

1. It is either by alphabetical or track number. Track number if that tag is present in teh ID3, alphabetical if it is not, or if you are playing the files from multiple sets of tags (picking media library>artist>all etc)

2. A good way to set up your audiobook is to rename the files as (chapter number<chapter number="">)-(<chapter name="">chapter name) or some such, If it's in a normal format (MP3, OGG, WMA, etc) you can also edit the tags to have track numbers.</chapter></chapter>

Q1 - In what order are files played in a folder? From googling around it seems like the order depends on the ID3 tags, but which tags exactly? Is it track number, or song title (in alphabetical order?).

If you play by folder, play sequence is file name in the alphabetic order. It's just like browsing a folder on a computer screen and playing from top to bottom. I am looking at a folder as I type this now. I put multiple CDs in it. All tracks have ID3V2.3 or ID3V2.4 tags with duplicating track numbers. But they play in the alphabetic order.

Yes it does alphabatize by track/file name (if browsing folders or multiple albums/artists) or track number (by Id3 tags in single album view)

For example if you browse folders and your tracks are named <tracknumber> - <trackname> then it will be all tracks with 01 as their track number first, then the 02s, etc. If browsing by ID3 tags it goes by track number only when viewing a single album, otherwise it is all alphabetical. When going by Id3 tags "the" is discounted in artist names (IE: The Doors would appear between Eagles and Cake) in folders "the" is not discounted in names (so The Doors would appear with The Cars, The Beatles, The Who, etc.)